Can I limit what they take. Or do I have no control. ?
IE. Can they / will they just drain my batteries at will. ?
Joining octopus flux export soon.
That isn't how it works 
Flux is just a tariff that gives people who have energy storage in their homes a financial incentive to push it into the grid at peak demand time. Essentially, Octopus are paying people a small amount of money to offer a small amount of grid balancing.
There are systems like the Givenergy where you can choose to have the system charge and discharge automatically in as economical a way as possible. However with the Foxess kit, no such integration exists, ie you are obliged to control it manually* - and hence cannot be forced into draining your batteries, or even discharging them at all, if you don't want to.
*If you have your inverter connected to Home Assistant via the rs485 link, you can do whatever automation you like.
Flux is just a tariff that gives people who have energy storage in their homes a financial incentive to push it into the grid at peak demand time. Essentially, Octopus are paying people a small amount of money to offer a small amount of grid balancing.
There are systems like the Givenergy where you can choose to have the system charge and discharge automatically in as economical a way as possible. However with the Foxess kit, no such integration exists, ie you are obliged to control it manually* - and hence cannot be forced into draining your batteries, or even discharging them at all, if you don't want to.
*If you have your inverter connected to Home Assistant via the rs485 link, you can do whatever automation you like.